1. Field of the Invention
The present invention relates to a method for adjusting the capacitance value of a capacitor that is built in a multilayer ceramic substrate, a multilayer ceramic substrate, and a method for manufacturing the same, and more particularly, to a method for adjusting the capacitance value of a built-in capacitor by laser trimming.
2. Description of the Related Art
In information processing apparatuses, such as a mobile communication terminal and a personal computer, for example, an increase in information processing speed, miniaturization of the apparatus, and improvement of multifunctionality have been actively pursued, and the improvement in the performance of the information processing apparatuses as described above has been achieved primarily by an increase in integration, an increase in processing speed, and an improvement of functionality of semiconductor devices, such as VLSI and ULSI.
However, even though the increase in processing speed and the improvement of functionality of semiconductor devices have been accomplished, signal delay, crosstalk, impedance mismatching, noise generation caused by fluctuation of power source output, and other problems may occur on a substrate on which devices are connected to each other. As a result, the system performance may be limited.
Accordingly, as an electronic component used in a high speed and high performance information processing apparatus, a multichip module (MCM: Multi-Chip Module) in which a plurality of semiconductor devices is mounted on a ceramic substrate has been used. A multilayer ceramic substrate is provided on which a wiring pattern for electrically connecting semiconductor devices in a three-dimensional manner is disposed, and alumina has been used for this type of multilayer ceramic substrate.
In a multilayer ceramic substrate using alumina, since a sintering temperature of an alumina powder is relatively high, such as about 1,500° C. or more, a high melting point metal, such as tungsten or molybdenum, must be used for an inner layer wiring pattern. However, since the high melting point metals as described above have a high specific resistance, signal loss increases, particularly in a high frequency region.
In order to solve this problem, a multilayer ceramic substrate has been developed which is formed by laminating a plurality of glass ceramic green sheets, followed by sintering. Since the glass ceramic green sheets are formed by adding a vehicle including a binder, a solvent, and other suitable ingredients to a ceramic powder and a glass powder, followed by mixing, and the sintering temperature is about 1,050° C. or less, simultaneous firing can be performed with a low temperature melting point metal, such as silver or copper, having a low specific resistance, and a multilayer ceramic substrate having a small signal loss in a high frequency region and superior electrical properties can be obtained.
In addition, recently, an attempt has been made in which a passive element, such as a capacitor, surface mounted on a multilayer ceramic substrate is built in a multilayer ceramic substrate so that the entire module is further miniaturized.
In a situation in which the element as described above is built in a multilayer ceramic substrate, when the electrical properties of the element are degraded as compared to that of a surface mounting element mounted on a substrate surface, the advantage obtained by a built-in element is reduced by approximately half. Thus, a built-in element must have properties equal to or greater than those of an element mounted on a substrate. Thus, a substrate base material is selected so that sufficient electrical properties of each built-in element can be obtained.
For example, in Japanese Unexamined Patent Application Publications Nos. 9-92983 and 2000-58381, a capacitor-built-in multilayer ceramic substrate is disclosed which has a structure in which a capacitor is built in, and a dielectric ceramic layer having a high dielectric constant is provided between capacitor electrodes so as to define the capacitor. As described above, when a dielectric ceramic layer having a high dielectric constant is provided between capacitor electrodes, even if the areas of capacitor electrodes are relatively small, a built-in capacitor having a high capacitance is provided.
The capacitance value of a capacitor built in a multilayer ceramic substrate may vary lot by lot because of, for example, variations in the thickness of the ceramic green sheets, variations in the printed area of the capacitor electrodes, variations generated when ceramic green sheets are laminated to each other, and variations in shrinkage which occurs when the ceramic green sheets are fired. Accordingly, in order to achieve a highly precise capacitance value of a built-in capacitor, it is necessary to perform trimming of a capacitor electrode with a laser or other suitable method. That is, by milling the area of a capacitor electrode using laser trimming, the capacitance value of a built-in capacitor can be set to a desired value.
However, when laser trimming is performed, a dielectric ceramic layer provided between capacitor electrodes is partially milled in addition to the capacitor electrode, and as a result, after the trimming is performed, the insulation resistance between the capacitor electrodes may be decreased and/or the Q value of the capacitor may be decreased in some cases.